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Derivative Financial Instruments - Additional Information (Details) £ in Millions	Sep. 30, 2019 USD ($) derivative	Dec. 31, 2018 USD ($) derivative $ / unit	Dec. 31, 2018 GBP (£) derivative $ / unit
Derivative [Line Items]
Number of derivative contracts with contingent feature in a net liability position | derivative	0
Other assets
Derivative [Line Items]
Derivative assets, gross	$ 700,000	$ 900,000
Other liabilities
Derivative [Line Items]
Derivative liabilities, gross	200,000	$ 1,400,000
Forward contracts
Derivative [Line Items]
Derivative, number of contracts | derivative		2	2
Cash collateral held from counterparty	21,600,000	$ 3,100,000
Cash collateral held at counterparty	0	$ 28,000,000.0
Collar contracts
Derivative [Line Items]
Derivative, number of contracts | derivative		2	2
Foreign contracts and collar contracts | Other assets
Derivative [Line Items]
Derivative assets, net	$ 21,600,000	$ 4,900,000
Designated as Hedging Instrument | Net Investment Hedging | Forward contract expiring 2024
Derivative [Line Items]
Derivative notional amount		450,000,000.0	£ 325.3
Designated as Hedging Instrument | Net Investment Hedging | Forward contract expiring 2021
Derivative [Line Items]
Derivative notional amount		$ 400,000,000.0	£ 285.8
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-call option | Minimum
Derivative [Line Items]
Derivative, option strike price (in dollars per pound sterling) | $ / unit		1.318	1.318
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-call option | Maximum
Derivative [Line Items]
Derivative, option strike price (in dollars per pound sterling) | $ / unit		1.448	1.448
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-put option | Minimum
Derivative [Line Items]
Derivative, option strike price (in dollars per pound sterling) | $ / unit		1.288	1.288
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-put option | Maximum
Derivative [Line Items]
Derivative, option strike price (in dollars per pound sterling) | $ / unit		1.535	1.535

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Derivative, Net Liability Position, Number of Instruments

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Fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, not subject to a master netting arrangement.

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Nominal or face amount used to calculate payments on the derivative liability.

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